1. Field of the Invention
The present invention relates to a piezoelectric ceramic laminate device, more specifically, to a piezoelectric ceramic laminate device suitable for actuators usable in precise positioning devices of machine tools, flow-controlling valves, automobile brakes, etc.
2. Description of the Background Art
Materials which mechanically strain when an electric field (voltage) is applied thereto are generally called piezoelectric materials or electrostrictive materials, and these kinds of materials are used as electromechanical transducers in many applications such as bimorphs, piezoelectric ignition elements, ultrasonic vibrators, piezoelectric buzzers, ceramic filters, etc.
Generally, when voltage is applied to a piezoelectric material, the crystal structure of the piezoelectric material is transformed and a maximum strain (maximum electromechanical coupling constant) is obtained at its crystal transformation point. Since the crystal transformation point varies depending on the composition of the piezoelectric material used, a proper piezoelectric material should be selected depending on a temperature at which it is used.
BaTiO.sub.3, Pb(Zr,Ti)O.sub.3 (which is generally called PZT), LiNbO.sub.3, LiTaO.sub.3, etc. are known as piezoelectric materials. Also usable are those obtained by adding other metal oxides to the above piezoelectric materials to improve various properties. Further, since a PNZST-type piezoelectric material which is obtained by partially substituting Zn in the above PZT piezoelectric material with Nb and Sn strains greatly without cracking or breakage, attempts have been made to use the PNZST piezoelectric material for actuators.
However, as schematically shown by a solid line in FIG. 15, the strain of the PNZST piezoelectric material suddenly increases almost to the maximum when a certain voltage (E.sub.q) is applied thereto. This means that, in practice, the strain of the PNZST piezoelectric material is controlled only at the maximum strain (.DELTA.Lo) and the minimum strain (0); in other words, the strain of the PNZST piezoelectric material is only controlled digitally. There also is a problem that a hysteresis loop of voltage vs. strain is relatively large in the PNZST piezoelectric material.
Although the above-mentioned digital control is effective in some cases, a piezoelectric material would be able to find much more applications in the field of actuators, if the analog control of the strain of the piezoelectric material can be performed, namely if the strain can be controlled at a certain point between the maximum and the minimum. To achieve the analog control of the strain, it is necessary to develop a piezoelectric material whose strain changes more gradually in proportion to the voltage applied and which has a strain/voltage hysteresis loop which is much slimmer. For instance, in the case of the PZT as shown in FIG. 16, .DELTA.X/X.sub.max should be small. Also, in the case of the PNZST as shown in FIG. 17, it is needed to have a strain changeable by a small .DELTA.L relative to the change of voltage .DELTA.E.